March 2000 Ž . Materials Letters 42 2000 371–379 www.elsevier.comrlocatermatlet Carbide formation mechanism during solidification and annealing of 17% Cr-ferritic steel I.M. Moustafa ) , M.A. Moustafa, A.A. Nofal ( ) Central Metallurgical Research and DeÕelopment Institute CMRDI , P.O. Box 87 Helwan, Cairo, Egypt Received 16 December 1998; received in revised form 6 August 1999; accepted 12 August 1999 Abstract Four heats of 17% Cr ferritic stainless were cast using an unidirectional solidification technique with two different cooling rates. Homogenisation annealing at 9008C, 10008C for 2 and 4 h holding time was carried out. The relation between the microstructure and hardness was studied. The cast structure consists of fine martensite, carbides and ferrite in rapidly cooled zone and of carbides and course ferrite in slow cooled one. After annealing at 10008C chromium carbides agglomerated and precipitated as a thin layer around the a-ferrite grains, which prevented the latter from coarsening. The obtained hardness decreased as the homogenisation temperature and time increased. This is probably due to the decrease of the total surface area of precipitates, i.e., carbide chains transformed to carbide layers. This in turn leads to the softening of the strained matrix. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction Chromium steel castings are used to resist corro- sion by aqueous solution aro near room temperature and by hot gases at elevated temperatures of 6508C. Chromium is the basic alloying element added to iron for obtaining passivity and does so when it is added in amount of or greater than 17%, which may wx be required in certain environments 1 . Carbon and chromium are so interrelated that when chromium is increased, carbon must be increased, if the steel is to wx retain its ability to be hardened by heat treatment 2 . Comprehensive development works on chromium steels belonging to the so-called 17% Cr group were w x done 3–7 . Homogenisation annealing for different ) Corresponding author. cast structures was performed, aiming at improving wx the homogeneity of microstructure 1 . Equilibrium diagram of iron 17% Cr, which gives a-ferrite and carbides at room temperature, is an attempt to recognize the phase transformation and its w x behaviour with temperature 8–11 . Different mi- crostructures along the axis of unidirectional solidi- fied ingot were obtained. The aim of this work is to study the behaviour of carbide formations and pre- cipitation in as-cast and annealed conditions. 2. Experimental work The investigated alloy with a chemical composi- tion shown in Table 1 was obtained using a unidirec- tional solidification technique explained elsewhere w x 12 . The equilibrium diagram of this alloy is repre- 00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0167-577X 99 00213-X